1. Field of the Invention
The present invention relates to a module (hereinafter referred to as a light emitting module) including a device (hereinafter referred to as a light emitting device) comprising an element sandwiching a light emitting material (hereinafter referred to as a light emitting element) between electrodes. In particular, the present invention relates to a light emitting module comprising a light emitting element (hereinafter referred to as an EL element) using a compound producing EL (Electro Luminescence) as the light emitting material. Here, an organic EL display and an organic light emitting diode (hereinafter referred to as an OLED) is included in the light emitting device in accordance with the present invention.
Further, the light emitting material used in the present invention includes all materials emitting light (phosphorescence and/or fluorescence) through singlet excitation, triplet excitation or both of them.
2. Description of the Related Art
In recent years, the development of an EL element using an organic compound (hereinafter referred to as an organic EL film) producing EL (Electro Luminescence) as a light emitting layer has advanced and EL elements using various kinds of organic EL films have been proposed. A flat panel display using such an EL element as a light emitting element has been developed.
A passive matrix type light emitting device and an active matrix type light emitting device have been known as a light emitting device employing an EL element. The passive matrix type light emitting device is a light emitting device employing an EL element with a structure in which an EL film is sandwiched between stripe-like anodes and cathodes provided to intersect at right angles. Also, the active matrix type light emitting device is a light emitting device in which each pixel has a thin film transistor (hereinafter referred to as a TFT) and the TFT connected to one of the anode and the cathode of the EL element controls a current flowing through the EL element.
The passive type light emitting device has an advantage that it has a simple structure which reduces manufacturing costs, but it has a problem that as the pixel is of higher definition (the number of pixels increases), the luminous intensity of the EL element needs to be increased, that is, a larger current is required with results of an increase in power consumption and a reduction in life.
On the other hand, in the active matrix type light emitting device, the pixel is capable of holding data because it is controlled by the TFT and the luminance of the EL element can be made constant irrespective of the number of pixels: that is, the luminance of the EL element can be reduced to a minimum as long as a user can see to prevent an increase in power consumption and a decrease in life.
From the above description, it is thought that the active matrix type light emitting device has smaller power consumption. However, because the active matrix type light emitting device is driven by a current, it is required to decrease the power consumption.
One object of the present invention is to provide a light emitting device having small power consumption and excellent visibility. Further, another object of the present invention is to provide an electrical appliance having a display section employing such a light emitting device and having small power consumption and excellent visibility.